Modified povray.h and pre-commit hook for new version numbering scheme.

For official proper releases, POV_RAY_PRERELEASE in povray.h should be undefined, resulting in the version number "X.Y.Z"; unix/VERSION should contain that same string.

For development versions, POV_RAY_PRERELEASE in povray.h should be set to "alpha.NUMBER", which the pre-commit hook will update to "alpha.TIMESTAMP", resulting in the version number "X.Y.Z-alpha.TIMESTAMP";
unix/VERSION should contain the string "X.Y.Z-alpha.NUMBER", which the pre-commit hook will also update to "X.Y.Z-alpha.TIMESTAMP";
if desired, "alpha." might be replaced with any other string; e.g. "beta.NUMBER" would result in the version number "X.Y.Z-beta.TIMESTAMP".

Note that the pre-commit hook script in .git/hooks in your local workspace needs to be replaced with the new version provided in "tools/git/hooks".